Flaky skin (fsn) is an autosomal recessive mouse mutation with papulosquamos disease features similar to human psoriasis. The aims of this proposal are to 1) isolate the gene that results in the mouse flaky skin (fsn) phenotype, and 2) to identify its human homolog. The fsn locus has been localized to an interval of approximately 3.3 cM, between D17Mit75 and D17Mit123 on distal mouse Chromosome 17. The fsn locus will be refined further by the generation of a high resolution intersubspecific intercross between BALB/cBy fsn/fsn mice and an inbred line Mus musculus castaneus. (CAST/Ei) and an intraspecific intercross with DBA/2J. We currently aim to generate a minimum of 1000 F2 progeny per cross, equivalent to 2000 meiotic events, capable of establishing a genetic linkage map with a resolution of <0.1 cM. We will complete the physical map of the region between D17Mit75 and D17Mit123 by isolating a series of overlapping YACs, BACs, and, if necessary, Pis. These will be used as reagents to generate additional polymorphic markers from the region that will be used to refine the region encompassing the fsn locus. Candidate genes will be isolated from the smallest co-segregating region using a variety of approaches: Direct screening of mouse cDNA libraries with YACs/BACs/PIs: Exon trapping: Direct selection with primary cDNAs from mouse skin, stomach, bone-marrow and lymph node. RT-PCR and northern blots will be employed to determine expression profiles and transcript sizes in fsn and wild-type (wt) mice and genes expressed in skin, forestomach and lymphoid tissue will be targeted first for mutational analysis. Genes will be extended to near full length and sequenced. Sequences in fsn and wt mice will then be compared. Large genes will be analyzed first nby indirect means such as SSCA, REF and CFLP. If no mutations are detected in coding sequence, genomic sequence of genes will be determined so that regulatory sequences can be examined. The isolation of the fsn gene will be further confirmed by performing immunohistochemistry with antibodies against the fsn gene product on cross-sections of fsn and wt adult mice to confirm is altered expression and/or distribution in fsn mice. Spatial and temporal modes of expression of fsn will be examined by immunohistochemistry and RNA in-situ hybridization to mouse sections from different developmental stages. The homologous human gene will isolated by screening the appropriate human cDNA library. This gene will be mapped and its involvement in psoriasis will be examined by screening for germ-line mutations in linked families and in sporadic cases, and with respect to protein distribution in lesional and normal skin.